1. Technical Field
This disclosure relates to an image forming apparatus.
2. Description of the Related Art
Some common image forming apparatuses such as printers, facsimile machines, copiers, plotters, and those having two or more of the functions of these apparatuses perform image forming (recording or printing) by causing recording liquid (hereinafter also referred to as “ink”) as liquid to adhere to a medium (hereinafter also referred to as “paper” or “paper sheet,” but not limited to paper in material; “medium to be subjected to recording,” “recording medium,” “transfer material,” and “recording paper” may also be used as synonyms) while conveying the paper, using, for example, a liquid discharger (liquid discharge device) including a recording head formed of a liquid discharge head that discharges liquid droplets of the recording liquid.
The term “image forming apparatus” means an apparatus that performs image forming by discharging liquid onto media such as paper, thread, textile, cloth, leather, metal, plastic, glass, wood, and ceramics. The term “image forming” means not only providing media with significant images such as letters, characters, and figures, but also providing media with insignificant images such as patterns. Further, the term “liquid” is not limited to recording liquid and ink, and may be any liquid as long as it becomes fluid when it is discharged. Further, the term “liquid discharger” means an apparatus that discharges liquid from a liquid discharge head, and is not limited to those performing image forming.
Known liquid discharge heads include those using a piezoelectric actuator formed of a piezoelectric element, those using a thermal actuator formed of a heat element, and those using an electrostatic actuator that generates an electrostatic force, as a pressure generation part (actuator part) for generating pressure to press ink, which is liquid, in an individual channel (hereinafter referred to as “pressure liquid chamber”).
The image forming apparatus has been required to output images of higher quality at higher printing rates. The number and the density of nozzles tend to increase in order to meet the former requirement. As a result, the distance between pressure liquid chambers tends to decrease, and the driving frequency for applying discharge energy tends to increase. With respect to the latter requirement, attempts have been made to elongate heads, and a full-line-type head capable of covering the entire area of a medium widthwise has been put into practical use.
In such a liquid discharge head required to have multiple nozzles at high density, the discharge energy applied to a predetermined pressure liquid chamber causes pressure variation or fluctuation of liquid in the pressure liquid chamber, and the pressure variation caused in the pressure liquid chamber also propagates to a common channel (hereinafter referred to as “common liquid chamber”) that supplies the liquid to multiple pressure liquid chambers.
If this pressure variation propagated to the common liquid chamber propagates back to the pressure liquid chamber discharging droplets of the liquid, the pressure variation varies the pressure of the pressure liquid chamber so as to prevent the pressure liquid chamber from discharging liquid droplets at a required droplet velocity with a required droplet amount (droplet volume), thus causing ejection failure (discharge failure). Further, if mutual interference, where the pressure variation propagated to the common liquid chamber propagates to an adjacent pressure liquid chamber to affect its liquid, occurs, leakage or discharge of liquid droplets from unintended nozzles and destabilization of a discharge condition are induced. As a result, a high-quality image is prevented from being output.
Therefore, as conventional examples of providing a vibration absorber in the common liquid chamber, Japanese Laid-Open Patent Application No. 7-171969 (Patent Document 1) discloses absorbing pressure in a common liquid chamber at the time of discharging ink by providing a foamed flexible material in the common liquid chamber, and Japanese Laid-Open Patent Application No. 2000-043252 (Patent Document 2) discloses providing a vibration absorber in a common liquid chamber or providing wedge-like projections in the common liquid chamber. Patent Document 2 also discloses providing a vibration absorber in the communication part between an ink pressure chamber and the common liquid chamber.
Further, as an example of providing a damper chamber that absorbs or releases pressure, Japanese Laid-Open Patent Application No. 8-20111 (Patent Document 3) discloses providing a single damper chamber that communicates with common liquid chambers through multiple communicating passages but does not communicates with the atmosphere; and filling the damper chamber with a compressible material for absorbing pressure variations due to pressure waves.
Japanese Laid-Open Patent Application No. 2002-103608 (Patent Document 4) discloses providing damper recesses in a first member different from a second member in which pressure generation chambers are formed with a diaphragm closing the opening of an ink reservoir chamber being provided between the first and second members; forming holes that communicate the damper recesses with the outside; and sealing the openings of the communicating holes with a film.
Japanese Laid-Open Patent Applications No. 2004-284196 (Patent Document 5) and No. 2005-125631 (Patent Document 6) each disclose forming, on a wall face of a common liquid chamber extending in an X direction in which multiple pressure liquid chambers are arranged, a damper surface of a pressure absorber that is lower in rigidity than the other wall faces and absorbs pressure through vibration; and not forming the damper surface entirely along the length of the common liquid chamber in the X direction so as to partially provide an area where the damper surface does not exist.
Japanese Laid-Open Patent Application No. 2004-299345 (Patent Document 7) discloses providing a free oscillation face formed of a thick-wall part and a thin-wall part as at least one of the wall faces of a common liquid chamber.
Japanese Laid-Open Patent Application No. 2005-119044 (Patent Document 8) discloses providing a member having rubber elasticity that absorbs pressure applied to liquid in directions other than the discharge direction because of partial deformation of the shape of a channel on at least a face of a wall of a reservoir that supplies the liquid to multiple channels which face comes into contact with the liquid.
In addition, Japanese Laid-Open Patent Application No. 2004-122428 (Patent Document 9) discloses providing a pressure damping partition wall formed of a low-rigidity material in the partition wall between pressure liquid chambers.
Japanese Laid-Open Patent Application No. 2003-311952 (Patent Document 10) discloses an inkjet head including a first flat plate layer formed of at least one flat plate, in which multiple nozzles for discharging ink and multiple pressure chambers communicating with the corresponding nozzles are formed; a second flat plate layer formed of at least one flat plate, in which a common ink chamber shaped to be elongated in a direction in which the pressure chambers are arranged; an ink channel having one end thereof communicating with each of the pressure chambers and having the other end thereof communicating with the common ink chamber; an ink supply passage connecting the common ink chamber and an ink supply source; a flat plate member in the form of a thin film positioned between the first flat plate layer and the second flat plate layer; and a damper chamber formed of a closed space in a flat plate facing the flat plate member on the side opposite to the common ink chamber.
Japanese Laid-Open Patent Application No. 2006-007629 (Patent Document 11) discloses an inkjet recording head in which multiple damper walls that deflect to absorb a pressure change of a common liquid chamber that supplies ink to individual pressure liquid chambers are formed in a wall that defines the common liquid chamber; and at least one of the damper walls is different in elasticity from the other damper walls.
Japanese Laid-Open Patent Application No. 2004-114315 (Patent Document 12) discloses providing a common liquid chamber with a damper mechanism for absorbing pressure.
Japanese Laid-Open Patent Application No. 2002-67310 (Patent Document 13) discloses stacking multiple members so that pressure generation chambers and a damper chamber are positioned on the same level and the pressure generation chambers and a common liquid chamber adjacent to the damper chamber are positioned on different levels, that the pressure generation chambers and the damper chamber have wall faces thereof formed of a diaphragm, and that the wall part between the common liquid chamber and the damper chamber is formed of an ink supply hole formation plate, in which ink supply holes for supplying ink from the common liquid chamber to the pressure generation chambers are formed.
However, in the case of providing a foamed flexible material or forming a damping structure in a common liquid chamber as disclosed in Patent Documents 1 and 2, there is difficulty in processing, and the cost of parts is high. For example, it is difficult to process and dispose the foamed flexible material. Further, as the driving frequency and the number of nozzles increase, the common liquid chamber pressure tends to increase, thus causing a problem in that it is difficult to ensure absorption and damping of the increasing pressure. Further, since the foamed flexible material is constantly in contact with the liquid in the common liquid chamber, the foamed flexible material is required to be highly resistant to liquid. This narrows the range of choices for material, which may lead to a further increase in the cost of parts. Further, according to the head disclosed in Patent Document 2, since the vibration absorber may be provided in the communication part between the ink pressure chamber and the common liquid chamber, the droplet discharge characteristic itself may be subject to variation.
Further, in the case of providing a damper chamber as disclosed in Patent Documents 3 and 4, it is necessary to perform processing to form the damper chamber, and the increase in part size causes an increase in the cost of parts. In particular, the damper chamber is filled with a compressible member such as air in Patent Document 3. However, controlling the amount of air in the damper chamber is itself difficult, and there is a problem in that if air separated from the damper chamber turns into bubbles to enter a pressure liquid chamber, it is impossible to sufficiently increase the pressure in the pressure liquid chamber, which may result in ejection failure or cause no liquid droplets to be discharged. Further, according to the head disclosed in Patent Document 4, since each ink reservoir chamber is formed on one side of the corresponding pressure generation chambers, and the damper recess parts are disposed next to the corresponding ink reservoir chambers with the diaphragm provided therebetween, it is difficult to ensure a large capacity for each ink reservoir chamber. In particular, in the case of an elongated head such as a line-type head, timely replenishment or supply may not be possible.
Further, an increasing pressure variation per unit time in a head can no longer be managed by forming, on a wall face of a common liquid chamber, a damper surface of a pressure absorber that is lower in rigidity than the other wall faces and absorbs pressure through vibration as disclosed in Patent Documents 5 through 7.
That is, if the pressure absorbing effect of the common liquid chamber is weak in the above-described configuration, as the instantaneous pressure variation becomes greater as in the case of high-frequency driving or discharging large droplets, a greater delay in supplying recording liquid into the common liquid chamber is caused by the pressure. This may prevent recovery of a meniscus so as to cause ejection failure.
Therefore, it is important to ensure early absorption of the pressure in the common liquid chamber in response to a pressure variation increase per unit time due to high-frequency driving. However, in the configuration where the damper surface of the common liquid chamber deforms and vibrates in order to absorb the pressure in the common liquid chamber, if the vibration of the damper surface is not completely damped, the vibration of the damper surface causes a pressure variation so that the meniscus does not completely recover at the time of discharging a droplet. This phenomenon makes it difficult to control a nozzle meniscus and causes undesirable variations in the volume, velocity, and discharge direction of a discharged droplet, thus preventing improvement of image quality.
This phenomenon no longer occurs after discharging is repeated in sequence, that is, after vibration is damped, because recording liquid is steadily supplied to normalize the operation of a meniscus. However, before the vibration of the damper surface is damped, the volume and/or velocity of a discharged droplet may slightly vary at the vibration period of the vibration of the damper surface so as to degrade image quality.
Further, according to the head disclosed in Patent Document 5, since a wall face of the common liquid chamber is formed of a damper surface of a pressure absorber, the area of the thin film part increases, in particular, in an elongated head such as a line-type head, it is difficult for the thin film to maintain rigidity as a part the same as in the head disclosed in Patent Document 11, which leads to a decrease in assembly ability.
Further, in the case of providing a member having rubber elasticity on at least a wall of a reservoir that supplies liquid to multiple channels as disclosed in Patent Document 8, a longer time is necessary before the vibration of the wall face is damped because of reception of a repulsive force generated by the rubber-elasticity member in addition to the above-described problem in the case of absorbing a pressure variation in the common liquid chamber through the vibration of a damper surface. As a result, the volume and/or velocity of a discharged droplet slightly varies at the vibration period of the vibration of the wall face, thus degrading image quality.
According to the inkjet head disclosed in Patent Document 10, the damper (buffer) chamber facing the thin film serving as a wall face of the common liquid chamber absorbs a pressure variation caused in the common liquid chamber.
However, since this damper chamber is closed, an elongated head such as a line-type head particularly has a problem in that a sufficient buffer effect is not produced for a relatively large pressure variation caused in the case of applying energy to multiple pressure liquid chambers, thus causing unstable discharge.
Further, according to the inkjet recording head disclosed in Patent Document 11, part of a wall face of a common liquid chamber is formed of a thin film so as to relax a pressure variation caused in the common liquid chamber the same as in Patent Document 10, but Patent Document 11 is different from Patent Document 10 in that the thin film directly faces the atmosphere and a closed space like the damper chamber is not provided. According to this configuration, it is possible to regard the atmosphere as having an infinite size with respect to the volume of the common liquid chamber, which is sufficient for absorbing pressure variation.
However, since the surface of the thin film has to be in contact with the atmosphere according to this configuration, there is the problem of a greater number of layout restrictions. Further, since the thin film is exposed, there is a problem in that a recording medium and the inkjet recording head may contact each other for some reason (such as a jam) to damage the thin film, thereby causing an outflow of liquid in the common liquid chamber. Particularly, in an elongated head such as a line-type head, the thin film has a large area so that it is difficult for the thin film to maintain rigidity as a part, which leads to a decrease in assembly ability.
According to the head disclosed in Patent Document 12, providing the damper mechanism for absorbing pressure in the common liquid chamber makes the assembling process of the head complicated.
Further, according to the head disclosed in Patent Document 13, the number of parts increases since the wall part between the common liquid chamber and the damper chamber is formed of the ink supply hole formation plate, in which the ink supply holes for supplying ink from the common liquid chamber to the pressure generation chambers are formed.